PROJECTSUMMARY This application is to continue our studies examining the role of von Willebrand factor (VWF) in the pathophysiologyofsicklecelldisease(SCD).DuringthecurrentfundingperiodwehaveshownthatinSCDa) theplasmaconcentrationofVWFisveryhighandthemoleculeishyperadhesive;b)transplantationofmouse SCDbonemarrowintoADAMTS13?/?miceresultsinspontaneousvaso?occlusivecrisis;c)SCDpatientplasma is able to activate endothelial cells to release VWF; d) plasma ADAMTS13 is defective in cleaving multimeric VWFandVWFstrings,butnotasmallpeptidesubstratesuchasthatcurrentlyusedintheclinicalADAMTS13 activity assay; e) VWF from SCD plasma shows less ADAMTS13?mediated proteolysis and more methionine oxidation than control VWF; f) high?dose intravenous infusion of N?acetylcysteine (NAC) decreases VWF multimersize,reducesdensecellformationanderythrocytefragmentation,andincreasestheconcentrationof plasma small molecule thiols. We also have strong evidence that the self?association of VWF is a very important factor in determining its functions, that VWF self?association is enhanced in SCD, and that the processisdecreasedbyplasmahigh?densitylipoprotein(HDL)andacceleratedbythrombospondin?1(TSP?1). We plan to continue our studies of the role of the VWF?ADAMTS13 axis with the following specific aims. SpecificAim1:Toidentifythemolecularlesion(s)responsiblefordefectiveADAMTS13cleavageofmultimericVWFin sicklecelldisease.WehavefoundthatADAMTS13proteolysisofmultimericVWFisdefectiveinSCDandwill investigateseveralpossibilitiesforthisdefect.SpecificAim2:ToevaluatetheeffectsofTSP?1orApoA?Ideficiency, orApoA?Ioverexpression,onthecourseofdiseaseinsicklemice.Here,wewilltestwhethergeneticconditionsthat eitherdiminishorworsenVWFadhesiveactivityalterthecourseofSCDinmice.SpecificAim3:Toevaluatethe effect on the course of SCD in mice of treatment with ADAMTS13, ApoA?I, or VWF, or with both ADAMTS13 and ApoA?I.ThesestudieswillcomplementthoseofSpecificAim2toexplorewhethertreatmentsthatalterVWF adhesiveactivitywillaffecttheacuteorchronicmanifestationsofSCD. WeexpectthesestudiestoprovidefurtherbiologicalinsightsintotherolesinSCDpathophysiologyplayedby theVWF?ADAMTS13axisandoxidativestress.WealsoanticipatethatwewilldiscovernewtargetsforSCD therapy.